1. Field of the Invention
The present invention relates to a system and a method for quality of service management for a partitioned storage device or subsystem.
2. Background Art
Data recording technologies continually increase the areal bit density available for single actuator storage devices. For conventional high performance, multi-user applications (or subsystems) the concentration of more data under fewer actuators challenges the usability of the capacity of the storage devices. The access rate specified for a multi-user subsystem can depend on a number of factors. The factors that determine the access rate can include the subsystem configuration, the configuration and transaction load of applications that access the subsystem, the degree of successful data caching between the application transaction and the subsystem storage device, and the amount of data stored under each actuator in the subsystem.
The specified access (or I/O performance) rate in connection with the access rate capacity of each storage device in the subsystem determines how many devices (or actuators) are to be implemented to satisfy the access rate specifications. In many conventional storage subsystems the minimum device configuration is determined by the number of actuators that are to be implemented. When the minimum device configuration is determined by the number of actuators, the subsystem can have excess storage capacity over what is required to meet the access rate specifications.
In many conventional storage subsystems, the replacement of older disk storage with new higher density disk storage is implemented via “volume folding” where the contents of smaller (older) storage devices are “folded” into a single storage device. Mapping a number of virtual partitions to a single physical partition is an example of volume folding. However, to operate within the subsystem access rate specifications the volumes that are folded into the single volume can not have a total access demand that exceeds the access capacity of the replacement single volume.
One conventional attempt at improving the performance of storage subsystems is storing frequently accessed data close to the center of the actuator stroke and the less frequently accessed data at the extremes of the actuator stroke. A data storage device can be virtually partitioned to match the actuator stroke to estimated dataset access densities. The access density is a measure of the relative number of virtual I/O's mapped onto physical I/O's. However, when the estimated dataset access densities are less than the actual dataset access densities, conventional data storage devices and data storage subsystems can experience difficulty managing device and subsystem partitioning and data storage operations can fail to operate in an efficient manner. Estimated access densities in conventional storage subsystem approaches are often exceeded due to application configuration changes, transaction load increases, and loading, reloading and backing up datasets that have low expected access density. The conventional subsystem administrator (or controller) may be unable to utilize excess capacity under the actuator while maintaining adequate system performance during higher demand. Overall performance of conventional data storage devices and data storage subsystems can be suboptimal.
Thus, there exists a need for a storage subsystem that utilizes excess storage capacity without degrading the access rate, optimizes the access density, and provides specified quality of service to all of the storage partitions.